Nanoporous membranes continue to emerge as important tools for the filtration and separation of nanoscale materials, processes which are strongly influenced by the size, shape, and surface chemistry of the nanopores. In particular, controlling the size, shape, and surface chemistry of nanopores in polymer membranes can significantly impact transport of molecular or ionic species through these membranes. If the pores are sufficiently small, the electrochemical double layer formed on the interior surfaces of these pores can be manipulated so as to control the transport of charged species through the membrane, facilitating charge-mediated filtration, sensing or even energy harvesting. See J. Cervera et al., Europhys. Lett. 71, 35 (2005). For example, the ability to gate and release these concentration gradients is integral to the development of an energy source. See J. Cervera et al., Electrochim. Acta 56, 4504 (2011). By altering the shape of the nanopore, the transport properties of the pore can be further tailored. Conically shaped nanopores, for instance, have been shown to rectify ionic currents through nanoporous polymer membranes. See C. Kubeil and A. Bund, J. Phys. Chem. C 115, 7866 (2011); P. Apel et al., Nucl. Instrum. Meth. B 184, 337 (2001); Z. Siwy et al., Phys. Rev. Let. 94, 048102 (2005); and Z. Siwy et al., J. Am. Chem. Soc. 126, 10850 (2005). Given a sufficiently small pore tip, these conical membranes can act as charge filters, allowing for increased discrimination when filtering charged species, be it ions, particulates, or biological media.
Controlling the size and shape of these pores with nanoscale resolution, however, is technically challenging. Plasma etching has been shown to create conically shaped pores; however, this technique requires expensive vacuum equipment, and can be difficult to adapt for large-scale application. See N. Li et al., Anal. Chem. 76, 2025 (2004). A common alternative method used to shape the commercially available nanopores into cones involves placing the membrane between a concentrated basic (alkaline) solution and an acidic solution, while applying up to 30 V across two electrodes, one on each side of the membrane. See P. Apel, Radiat. Meas. 34, 559 (2001); C. C. Harrell et al., Small 2, 194 (2006); P. Scopece et al., Nanotech. 17, 3951 (2006); and J. E. Wharton et al., Small 3, 1424 (2007). The basic solution etches the membrane, while the acidic solution neutralizes any etchant that diffuses through the membrane. Because the nanopore density is small, the solution resistance through the membrane is exceedingly large and dominates the response of the electrochemical cell. The potential difference applied across the membrane creates an energetic barrier, such that it is energetically less favorable for the hydroxide ions to travel through the pores. The resulting concentration gradient produces asymmetric chemical etching of the polymer membrane, ultimately creating conical pores. See C. C. Harrell et al., Small 2, 194 (2006). Typically, these materials have been used for fundamental research studies examining the electrical behavior of single conical nanopores and nanoporous membranes with low pore densities. See J. Cervera et al., J. Chem. Phys. 124, 104706 (2006); N. Li et al., Anal. Chem. 76, 2025 (2004); P. Ramirez et al., Phys. Rev. E 68, 011910 (2003); and M. R. Powell et al., Nat. Nanotech. 6, 798 (2011).
At high nanopore densities, however, the solution resistance through the membrane becomes quite small and is insufficient to dominate the electrochemical response of the cell. Here the voltage across the membrane is only attributed to IR drop as dictated by Ohm's Law. To apply the same potential difference across the membrane, needed to induce the etching asymmetry, a larger current is required. To supply this current Faradaic processes are needed at the electrodes, which at more than 1.5 V, include the production of potentially hazardous hydrogen and oxygen gases. At 30 V, the production of hydrogen and oxygen gases and the subsequent change in solution pH make this voltage-based process ill-suited for the conical shaping of high density nanopores. Therefore, a need remains for an effective, tunable, inexpensive, and safe alternative to this process.
Nanopores have been used to successfully control ionic transport through inorganic and polymeric membranes. See C. R. Martin et al., Adv. Mater. 13, 1351 (2001); and C. R. Martin et al., J. Phys. Chem. B 105, 1925 (2001). Unlike many biological systems where ions are passed through channels of exquisitely arranged functional groups, nanopores rely on the overlapping electrochemical double layer, formed by the nanopore walls and solution, to electrostatically control ion movement through the pore. See Z. Siwy et al., Phys. Rev. Let. 94, 048102 (2005). Nanopore surface charge and solution concentration dictate the electrical field strength and size of the double layer, while the nanopore diameter controls overlap of the double layer inside the nanopore. See P. Ramirez et al., Phys. Rev. E 68, 011910 (2003); and Z. Siwy et al., J. Am. Chem. Soc. 126, 10850 (2005). Control over nanopore shape to include asymmetric geometries, such as cones, enables the creation of ionic diodes. See C. Kubeil and A. Bund, J. Phys. Chem. C 115, 7866 (2011); J. Cervera et al., Electrochim. Acta 56, 4504 (2011); P. Apel et al., Nucl. Instrum. Meth. B 184, 337 (2001); N. Li et al., Anal. Chem. 76, 2025 (2004); L. J. Small et al., RSC Adv. 4, 5499 (2014); W.-J. Lan et al., J. Am. Chem. Soc. 133, 13300 (2011); D. Momotenko and H. H. Girault, J. Am. Chem. Soc. 133, 14496 (2011); and J. P. Guerrette and B. Zhang, J. Am. Chem. Soc. 132, 17088 (2010). Once fabricated, nanopore shape is fixed, allowing only for solution concentration and surface charge to be easily varied. Generally, a lower salt concentration in solution will lead to a larger double layer. For a given nanopore size and fixed surface charge, this will create more overlap of the double layer inside the nanopore and result in increased ionic selectivity.
The ability to control whether cations or anions are selectivity transported is determined by the surface charge present on the nanopore walls. As shown in FIG. 1A, a positive surface charge allows the selective transport of anions, such as Cl−, whereas a negative surface charge allows the transport of cations, such as Na+, as shown in FIG. 1B. Further, for conical pores, selectivity is controlled by concentration at the pore tip. Surface charge in chemically active nanopores such as polycarbonate can be manipulated by the pH dependent protonation of the carbonate groups. See Z. Siwy et al., J. Am. Chem. Soc. 126, 10850 (2005). Other nanopore materials, such as gold, may easily be modified by chloride adsorption to place a negative charge to the surface. See C. R. Martin et al., Adv. Mater. 13, 1351 (2001). An applied voltage, however, remains the most attractive triggering mechanism, allowing surface charge to be changed with the flip of a switch. See M. Powell et al., Nat. Nanotech. 6, 798 (2011); E. B. Kalman et al., Anal. Bioanal. Chem. 394, 413 (2009); J. Elbert et al., Adv. Funct. Mater. 24, 1591 (2014); and F. Buyukserin et al., Small 3, 266 (2007). Therefore, a further need remains for a method to reversibly or irreversibly electrochemically switch the pore surface between multiple chemically stable states without the need for a continuously applied gate voltage to retain nanopore selectivity.